The present invention relates generally to digitizing tablets, and more particularly to a method and apparatus for eliminating an offset signal in an electrostatic digitizing tablet or panel associated with a computer.
Electrostatic digitizing panels having a resistive layer covered with a non-conductive plate such as glass, are known in the art. When operating in a touch mode, the corners of the resistive layer are typically stimulated with an AC signal so that when an object such as a user's finger touches the non-conductive plate, the finger capacitively couples to the resistive layer thereby causing current to flow in the corners of the resistive layer. The position of the user's finger relative to the digitizing panel can typically be determined by calculating a ratio of current flowing in each of the corners of the resistive layer.
One problem with the known electrostatic digitizing panels is their susceptibility to parasitic capacitance. In particular, metal components which are grounded to the system ground potential of the computer, surround the digitizing panel. The metal computer components can capacitively couple to the resistive layer of the digitizing panel which results in an AC (alternating current) offset signal or current flow through each of the corners of the resistive layer. The resulting AC offset current or signal is not stable and tends to vary with temperature, time, humidity etc. The AC offset signal may be comparable or even stronger than the signal level attributable to a user's finger.
In an idle state of the electrostatic digitizing panel (e.g. when a user's finger is not touching the non-conductive plate), the AC offset current flow in the corners of the resistive layer is due only to the capacitive loading caused by the surrounding metal computer components. The AC offset current is also present in an active state of the digitizing panel when a user's finger is touching the non-conductive layer. In the active state, the AC offset current disadvantageously adds to the current flow attributed to the user's finger which can cause errors in determining the position coordinates of the user's finger relative to the digitizing panel.
Thus, in order to accurately determine the position of a user's finger relative to the digitizing panel, the AC offset signal must be compensated for, or otherwise subtracted from the signal attributable to the user's finger. What is needed therefore is an electrostatic finger-sensitive digitizer that compensates for, or otherwise cancels an offset signal generated as a result of stray capacitive coupling between a resistive layer of a digitizing panel and grounded metal components of the computer.